Inductively coupled plasma reactors are used in processing semiconductor wafers or other workpieces. Inductively coupled plasma reactors are currently used to perform various processes on semiconductor wafers including metal etching, dielectric etching, and chemical vapor deposition, for example. In an etch process, one advantage of an inductively coupled plasma is that a high density plasma ion density is provided to permit a large etch rate with a minimal plasma D.C. bias, thereby permitting more control of the plasma D.C. bias to reduce device damage. For this purpose, the source power applied to the antenna and the DC bias power applied to the wafer pedestal are separately controlled RF supplies. Separating the bias and source power supplies facilitates independent control of ion density and ion energy, in accordance with well-known techniques. Typically, to produce an inductively coupled plasma, the inductive antenna is provided outside the reactor chamber adjacent a wall or walls capable of allowing inductive power coupling into the reactor chamber. The inductive antenna provides the RF power which ignites and sustains the plasma.
One characteristic of such a plasma reactor is that the spatial distribution of the plasma ion density across the wafer surface is critical to uniform processing across the workpiece. In a metal etch process, for example, because the etch rate is affected by plasma ion density, non-uniform plasma ion density across the workpiece results in non-uniform etch rate across the wafer. As a result, the etch process is difficult to control and may over-etch devices on some portions of the wafer and under-etch devices on other portions of the wafer, leading to reduced production yield.
Hence, the spatial distribution of the plasma ion density within the chamber must be precisely controlled to optimize processing. The geometry and placement of the inductive antenna along with the shape of the reactor chamber in large part determines the spatial distribution of the plasma ion density within the reactor chamber. Thus, antenna and chamber arrangement are optimized to ensure plasma uniformity across the workpiece.
To accomplish this, the antenna typically is placed adjacent only a portion of a chamber wall. This necessarily results in uneven power coupling through different portions of the chamber walls, producing areas of high and low coupling through the reactor walls. FIG. 1A illustrates one reactor configuration optimized to provide plasma uniformity across the workpiece in accordance with U.S. Pat. No. 5,753,044, by Hanawa et al., entitled RF PLASMA REACTOR WITH HYBRID CONDUCTOR AND MULTI-RADIUS DOME CEILING, issued on May 19, 1998, herein incorporated by reference in its entirety. With this configuration, a coil antenna 20 may be located adjacent a portion of a multi radius dome-type top wall 110 of reactor 100 to provide uniform processing across a workpiece 30. In such a reactor, the top wall 100 may be formed of a dielectric material such as aluminum oxide, quartz; ceramic, or other material capable of coupling inductive power.
One problem with such reactors is that the fabrication process leaves deposits on the reactor chamber surfaces. These deposits can contaminate the process environment and reduce yields. This particularly is true as the size of the devices fabricated on the workpiece are reduced. Thus, these deposits periodically must be removed to provide acceptable process yields.
One method of removing contaminates is to dry clean the chamber using a plasma. This may be done simultaneously with workpiece processing as disclosed in U.S. Pat. No. 5,817,534, by Ye et al., issued Oct. 6, 1998, entitled RF PLASMA REACTOR WITH CLEANING ELECTRODE FOR CLEANING DURING PROCESSING OF SEMICONDUCTOR WAFERS; and in U.S. Pat. No. 5,879,575, by Tepman et al., issued Mar. 9, 1999, entitled SELF-CLEANING PLASMA PROCESSING REACTOR; or by generating dry clean plasma after workpiece processing is complete as disclosed in U.S. Pat. No. 5,514,246, by Blalock, issued May 7, 1996, entitled PLASMA REACTORS AND METHOD OF CLEANING A PLASMA REACTOR; all herein incorporated by reference in their entireties. In implementation, however, these solutions do not always allow the desired optimization of plasma uniformity across the workpiece.
Another problem which can occur in reactors optimized for plasma uniformity across the workpiece is that, due to the uneven power coupling through different portions of the chamber walls, deposits can form non-uniformly across the interior surface of the chamber wall. For example, FIG. 1B shows a typical distribution of deposits on the interior surface 10 of the top wall 110 for the reactor configuration of FIG. 1A. More deposits form on portions 10a and 10c of the interior surface 10 of the top wall 110 where the antenna coil does not confront the chamber than where it does. Further, under most operating parameters, no deposits will form on the portion 10b of interior surface corresponding to location of the coil antenna 20 (shown in FIG. 1A).
In such instances observed by the present inventors, generating a dry clean plasma using the source power coil can cause undesirable results. For example, the dry clean plasma cleans most efficiently opposite the coil antenna, the location with the least amount of deposits. Further, because inductive coil antennas also couple a small amount of capacitive power, the portion 10b of the interior surface opposite the antenna may sputter during the cleaning process. In some dry cleaning processes sputtering and subsequent redeposition of top wall 110 material can significantly inhibit, or even prevent dry cleaning of some deposits. For example, using a fluorine based plasma to dry clean an aluminum oxide wall can lead to aluminum fluoride deposits over portions 10a and 10c. The deposited aluminum fluoride forms a sandwich structure with the existing deposits thereon, which inhibits their removal and can lead to flaking of the sandwich structure.
Thus, what is needed is a reactor that is both optimized to ensure plasma uniformity across the workpiece, and capable of providing more efficient cleaning of reactor chamber surfaces.
In an embodiment of the plasma reactor of the present invention, a chamber adapted to process a workpiece having at least one wall capable of allowing inductive power coupling into the reactor chamber is provided. A source power antenna, capable of generating a processing plasma, confronts a portion of the at least one wall. A dry clean antenna is located adjacent the chamber beside a portion of the at least one wall not confronted by the source power antenna.
In the preferred method of the present invention, the source power antenna generates a processing plasma for processing a workpiece. During workpiece processing, the dry clean antenna does not substantially interfere with uniformity of a processing plasma across the workpiece. In preferred embodiments, the dry clean antenna is provided with essentially a floating potential. As such, in more preferred embodiments, the dry clean antenna does not have any voltage applied, but instead is allowed to float so as not to significantly interfere with coupling of source power provided by the source power antenna. After workpiece processing has ceased, a dry clean plasma may be generated by inductive coupling using the dry clean antenna.
The reactor configurations of some more preferred embodiments allow dry clean power to couple into the chamber away from the source power antenna, where greater amounts of deposits typically accumulate. Some embodiments of the present invention may provide increased cleaning plasma density more remote from the source power antenna. This allows for improved cleaning of not only remote portions of the inductive coupling wall of the reactor, but also provide improved cleaning of other remote regions, such as the pumping apparatus and its associated components, for example. Further, the reactor configuration of some more preferred embodiments allow less power to be coupled near portions with less accumulation. As such, embodiments of the present invention allow optimization of the dry clean plasma to improve dry clean effectiveness.
In some embodiments, the source power antenna also may couple power to the dry clean plasma, preferably in parallel with the dry clean antenna. With such embodiments, the source power antenna may be operated so that it couples less power to the dry clean plasma. As such, sputtering of the chamber wall under the source power antenna can be inhibited, thus both reducing the flaking of deposits caused by chamber wall contaminates during the cleaning process, and improving the longevity of the chamber wall.
In one of the preferred embodiments, an inductive coil source power antenna is disposed over a reactor with a multi-radius dome top wall. The source power coil antenna is located adjacent a portion of the top wall and a dry clean coil antenna is nested within the source power coil antenna. A switching circuit may be employed to couple RF power in the form of processing plasma source power to the source power antenna, while allowing a floating potential for the dry clean antenna, during workpiece processing. Also, it may couple RF power in the form of dry clean power to the dry clean antenna, as well as, the source power antenna during the dry clean process.